Known processes for preparing 1,4-butanediol include a butadiene process comprising acetoxylating butadiene with acetic acid and oxygen, hydrogenating the product to obtain diacetoxybutane, which is hydrolyzed to 1,4-butanediol (see JP-A-52-7909, JP-A-52-65208, JP-A-52-65209, and JP-A-52-133912, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a maleic anhydride process comprising hydrogenating maleic anhydride to produce 1,4-butanediol and .gamma.-butyrolactone (see JP-A-2-233627), and an acetylene process comprising reacting acetylene and formaldehyde and hydrogenating the resulting 1,4-butenediol (see JP-A-52-91813).
The crude 1,4-butanediol obtained in these processes is usually purified by distillation, but it is difficult to achieve a high purity through simple distillation because the crude 1,4-butanediol contains impurities whose boiling points are extremely close to that of 1,4-butanediol and impurities which form azeotropic mixtures with 1,4-butanediol. Therefore, in order to increase the purity, large-sized equipment or high energy has been required for purification.
Of the impurities contained in crude 1,4-butanediol, 1,2-diacetoxybutane (hereinafter abbreviated as 12DAB), 1,4-diacetoxybutane (hereinafter abbreviated as 14DAB), 1,2-hydroxyacetoxybutane (hereinafter abbreviated as 12HAB), 1,4-hydroxyacetoxybutane (hereinafter abbreviated as 14HAB), dibutylene glycol (hereinafter abbreviated as DBG), 2-(4'-hydroxybutoxy)tetrahydrofuran (hereinafter abbreviated as BGTF), 2-(4'-oxobutoxy)tetrahydrofuran (hereinafter abbreviated as BDTF), and 1,4-di-(2'-tetrahydrofuroxy)butane (hereinafter abbreviated as BGDTF) cause coloring, cutting, and the like disadvantages when 1,4-butanediol is made into resins, fiber, etc.
In order to remove these impurities, JP-A-61-197534 proposes a method consisting of hydrogenation of crude 1,4-butanediol, and JP-A-6-172235 teaches a method comprising removing high-boiling point components by distillation followed by hydrogenation. According to these methods, however, the hydrogenation treatment must be followed by re-distillation to separate the impurities, requiring large-sized equipment or high energy for purification as mentioned above. Therefore, the methods are not always deemed efficient.
While not all the impurities present in crude 1,4-butanediol obtained by the conventional processes, such as a butadiene process, a maleic anhydride process, and an acetylene process, have been made clear as to the structure or the mechanism of formation, it has been proved that BGTF, BDTF, and BGDTF, especially BGTF, are re-produced during a distillation step. That is, as long as distillation is employed for the purification of crude 1,4-butanediol, there is a limit, of necessity, in removal of impurities, particularly BGTF. Continuous operation makes it more difficult to remove the impurities.
The outstanding problems of the process for preparing 1,4-butanediol are elaborated below, taking the butadiene process for instance.
The conventional butadiene process starts with reaction of butadiene with acetic acid and oxygen-containing gas, followed by hydrogenation to obtain diacetoxybutane (see FIG. 1). As shown in FIG. 2, the resulting diacetoxybutane is hydrolyzed, and acetic acid and water are distilled off from the liquid reaction mixture containing crude 1,4-butanediol (hydrolyzate) in a first distillation tower. Diacetoxybutane and hydroxyacetoxybutane are separated from the hydrolyzate in a second distillation tower. The resulting crude 1,4-butanediol is hydrogenated with hydrogen gas in the presence of a catalyst for hydrogenation. After tetrahydrofuran (hereinafter abbreviated as THF) is concentrated in a third distillation tower, high purity 1,4-butanediol is recovered from a fourth distillation tower as a side stream or a bottom.
Although this process provides 1,4-butanediol having a very high purity, it has many problems to be solved, such that diacetoxybutane and hydroxyacetoxybutane must be separated twice, the conditions of the separation are extremely severe, a considerable amount of THF is by-produced, and concentration of 1,4-butanediol or separation of high-boiling point impurities are conducted in three places.
A melt crystallization method is a technique used for increasing the purity of an organic compound having a high purity (e.g., 97 wt % purity) to a still higher level (e.g., 99.9 wt % purity). Industrial application of melt crystallization is found in the purification of bisphenol A and acrylic acid (see JP-A-7-163802 and JP-A-9-155101). Both the compounds have as low a viscosity as several centipoises at around their freezing point. On the other hand, 1,4-butanediol has as high a viscosity as 119 cp at around its freezing point (20.degree. C.). Therefore, it has been regarded very difficult to apply melt crystallization to purification of crude 1,4-butanediol. In fact, as far as we know, there is no report on application of melt crystallization to purification of a crude organic compound like crude 1,4-butanediol which has a relatively high content of impurities whose freezing points are considerably lower than that of the target organic compound and has a high viscosity (about 120 cp) at about its freezing point.